1. Field of the Invention:
The present invention relates to the field of corrugating adhesives, and particularly to a novel adhesive including fibers.
2. Description of the Prior Art:
Corrugated board is manufactured in a continuous process whereby, for example, a "medium" stock of paper is fluted with heat and pressure and then adhered to a "Kraft" liner a with starch-based adhesive. The resultant fluted medium and liner combination, known as a "single faced web", is pulled mechanically down to the "glue unit" where starch-based adhesive is applied to the exposed flutes. The web with adhesive is then combined with another "Kraft" liner and the resultant "double faced corrugated board" is dried out on the "hot plate section". Further processing of the combined board is achieved by in-line scoring, slitting and finally cutting to desired length. The sheets of combined board, a.k.a. corrugated board, are conveyed to "finishing equipment" for slotting, diecutting and/or printing.
The liners and the medium may be of various weights, measured as pounds of fiber per thousand square feet, and are manufactured from a combination of virgin and reclaimed fiber. Medium may in fact be manufactured from 100% reclaimed fiber. The strength of the paper is dependent on the amount of fiber per unit of paper and upon additives used in the manufacturing process. Because greater strength is required on the outside of a container, liners generally contain more strength additives than the medium contains. One of the most widely used additives is starch. Ranging from industrial unmodified corn starch to high priced specialty starches such as those with cationic, anionic, or amphoteric properties, starch is used in the manufacture of linerboard at the rate of 10 to 100 pounds of starch per ton of paper produced.
Historically, the liners and medium have been adhered in the manufacture of corrugated board with a mixture that contains:
1. Cooked Starch--a.k.a. carrier starch which is used for viscosity and tack properties; PA1 2. Uncooked Starch--a.k.a. raw starch for drying and additional bonding properties unattainable by additional cooked starch due to viscosity limitations; PA1 3. Caustic Soda--to assist in the cooking of the carrier starch and to lower the ultimate cooking temperature of the uncooked starch; and, PA1 4. Borax--converted to meta-borate by caustic soda and in this form complexes with the cooked starch for additional viscosity and cohesive strength. PA1 1. Carrier--A very discreet amount of cooked starch is responsible for the viscosity. The balance of the starch is uncooked. PA1 2. No-Carrier--All of the starch is slightly cooked or swollen with heat and caustic soda for viscosity. PA1 3. Carrier-No-Carrier--A discreet amount of cooked starch is responsible for about one-half of the viscosity. The rest of the viscosity is obtained by swelling the uncooked starch slightly.
The resultant adhesive has a consistency not unlike that of thin pancake batter. Measurable parameters include viscosity, gel point and percent solids. The viscosity is measured by a cup device with a hole in it which allows a certain amount of adhesive to pass through it in a measured unit of time. The number of seconds required is the viscosity reported. The gel point is the measured temperature at which the uncooked starch in the adhesive begins to absorb water, or cook, and rapidly increases in viscosity. A test tube about half full with adhesive is held by a clamp and immersed in boiling water; the adhesive is agitated with a glass thermometer. The viscosity rise will be very noticeable. The temperature of occurrence is the gel point. The percent solids is simply the amount of total starch divided by the weight of the entire batch.
There are several varieties of unmodified and specialty starches available for enhanced performance on the corrugator. These products include starches which have been acid thinned to allow for additional cooked solids per batch while maintaining viscosity parity. Products of this type are known as specialty carrier starches. Other types of additives are used for cross-linking in the creation of a less soluble or insoluble glue line. These additives typically are either ketone-aldehyde or resorcinolic in nature. Boxes used for shipping fruit and vegetables are manufactured from these types of additives, sometimes in concert with a specialty carrier starch.
Several examples of corrugating adhesive formulations have existed in the prior art. The types of adhesive preparation include:
There are several varieties of equipment available for the preparation of starch adhesive. Some are completely automatic, some manual, others a combination of the two. The prior art equipment prepares adhesive as either a carrier, no-carrier or carrier-no-carrier.
The usual starch-making process for corrugated board involves the use of a carrier and a raw (secondary) portion. The cooked starch is called the "carrier" because it suspends the uncooked starch present in its natural granular form. The thoroughly dispersed carrier forms strong films, provides the necessary application viscosity, and governs an adhesive's rheology (flow properties) and water-holding capacity. The uncooked portion remains inert until applied to the flute tips of the medium. The heat from the corrugation process causes this raw starch to gelatinize, absorbing large amounts of water, thus increasing viscosity. This action also decreases the availability of water for the carrier, increasing its viscosity as well. Triggered by the cooking of the raw starch, the carrier forms a matrix that adheres to the paper, bridging the liner to the medium interface.
A number of patents are directed to starch-based adhesives. Many of these disclose various formulas for the adhesive, or differing procedures for preparing the adhesive or its constituents.
The Chiu, U.S. Pat. No. 4,329,181, and Ware, U.S. Pat. No. 4,343,654 describe particular methods comprising certain steps for preparing an adhesive. Modifications of the starch component are described in the Lane, U.S. Pat. No. 4,587,332, and Casey, U.S. Pat. No. 2,610,136.
The DiDominicis, U.S. Pat. No. 4,240,841, discusses the general adhesive art including the Stein-Hall two component aqueous system and the "no-carrier" system. This patent is directed to the inclusion of a chemical flow modifier to control viscosity. Similarly, the Smith, U.S. Pat. No. 3,004,855, describes a method for preventing loss of viscosity by treating the starch with chemical stabilizers. The Krankkala, U.S. Pat. No. 4,339,364, discusses an adhesive formulation which includes a thickener, such as hydroxymethylcellulose, and a cross-linking agent, such as ureaformaldehyde and glyoxal.